The present invention relates to impedance measurement and more particularly to the automatic measurement of linear impedances and the detection of non-linear electrical devices.
Electrical ohmmeters are frequently used to make measurements "in circuit" where various combinations of passive and semiconductor devices are interconnected. Many ohmmeters are capable of forward biasing semiconductor junctions so that measurement of resistance across such a junction becomes tedious. At least two readings must be taken even to determine that a junction is being forward biased and then a decision must be made as to which is the desired reading. Stated another way, conventional ohmmeters require active operator involvement in the determination of whether a non-linear device is present, leaving room for measurement error. Furthermore, evaluation of such a device usually requires switching to a different mode or range.
In accordance with the illustrated embodiment, the present invention automatically checks the device(s) under test for non-linearity and if a linear resistance measurement is impossible, an appropriate message is generated and the device forward voltage drop is displayed. If the device under test is linear, the most valid measurement will be displayed. As will be seen in the following discussion, the present invention makes the measurements automatically, thus minimizing both front panel and measurement complexity without the necessity of the operator needing to change modes or making the ultimate determination that the device is non-linear.
The present invention includes a method and apparatus for determining whether a device under test is linear or non-linear and for calculating the impedance of the device under test if it is linear. The device under test is serially interconnected with a linear device of a known value to form a voltage divider. A fixed D.C. voltage and a positively going square wave signal having an average value that is substantially equal to the value of the D.C. voltage are alternately applied to the voltage divider. For each of these applied signals, the average value of the voltage across the device under test is measured and their values compared. If the compared values are substantially different, then the device under test is identified as a non-linear device. If the compared values are substantially equal, then the device is linear and its impedance value may be calculated using the standard voltage divider formula since the value of linear device in the divider is known, as is the nominal value of the two applied signals, and the average value of the voltage across the device under test has been measured.